Universally, marine waterjet propulsion systems consist of rotating and stationary rows of blades. The rotating blade rows are termed impellers. The purpose of the rotating blade rows is to raise the total energy of the water passing through the propulsion system, which can be used to produce useful thrust to propel the host vessel through the water. The stationary blade rows are termed diffusers or guide vanes. One purpose of the stationary blade rows, if positioned downstream from a rotating blade row, is to recover rotational flow energy produced by the impeller that can be used to augment the ability of the propulsion system in producing useful thrust to propel the host vessel through the water. If the stationary blade row is positioned upstream from a rotating blade row, one of its purposes is to mitigate large-scale fluctuations in flow velocity magnitude and direction seen by a rotating blade row.
Fluctuations in the magnitude and direction of flow velocities can result in detrimental consequences to the performance of the propulsion system. Specifically, they can cause fluctuating pressures to occur on any effected blade row, rotating or stationary. These fluctuations typically result in reduced efficiency for the propulsion system, increased vibration, and increased noise. In the event of severe fluctuations, the resulting surface pressures on blade rows may be reduced to levels below the vapor pressure of water, causing the water to boil. That phenomenon is termed cavitation. Bubbles of water vapor are created on the surfaces of the blades, which may coalesce into large cavities that remain attached to the blades or which may be shed from the blade row surfaces to travel downstream. The cavities and bubbles are detrimental to the performance of the propulsion system in a number of ways.
If the extent of cavitation in the propulsion system is severe enough, blockage of the flow of water through the system occurs, resulting in cavitation breakdown, or thrust breakdown, where the useful thrust of the system is reduced catastrophically.
If the cavities or bubbles of water vapor find their way to a region of the system where pressures in the flow field are above the vapor pressure of water, rapid condensation of the water vapor occurs as the bubble implodes back into a liquid state. The action of that implosion is violent, resulting in excessive transient pressure fluctuations that are forceful enough to physically damage the structure of the propulsion system components.
Implosion of the water vapor bubbles will, at a minimum, result in volume and pressure fluctuations sufficient to generate noise. Additionally, these fluctuations result in vibration of the propulsion system structure as well as of the associated host vessel structure. The vibration can lead to fatigue failure of the structures, as well as their radiation of additional noise both within the host vessel and into the water.
Previously known waterjet propulsion systems suffer universally from cavitation. Typical installations of waterjet propulsion systems are such that fluctuations in the magnitude and direction of flow velocities cannot be sufficiently mitigated by any practical means without severely restricting the operating range of the host vessel. Over some range of vessel operating conditions, cavitation is severe enough to result in physical damage to the propulsion system or, at a minimum, in a significant increase in vibration and noise.